Display system using conjugate optics and accommodation features and method of displaying and viewing an image

ABSTRACT

A display system including a retro-reflector, and optical means including a projection lens and an accommodation feature for directing light from an image source to the retro-reflector, light reflected by the retro-reflector being provided for viewing, said optical means and retro-reflector being cooperative to establish a conjugate optics path including the accommodation feature along which the light is directed to the eye or eyes of a viewer for viewing of an image, the accommodation feature includes a conjugation lens means and it and the retro-reflector are cooperatively positioned such that as light passes back through the conjugation lens means along the same path it would have followed had the retro-reflector been placed at the image plane of the projection lens had the conjugation lens means not been in position. A servo system for moving the projection lens and the retro-reflector according to a transfer function also may be included.

This application claims priority under 35 USC 119(e) of U.S. ProvisionalApplication Ser. No. 60/025,755, filed Sep. 19, 1996.

CROSS-REFERENCE TO RELATED APPLICATIONS

Copending U.S. patent applications Ser. No. 08/295,383, filed Aug. 24,1994, now U.S. Pat. No. 5,621,572 entitled "Optical system for a headmounted display using a retro-reflector and method of displaying animage" and Ser. No. 08/383,466, filed Feb. 3, 1995, now U.S. Pat. No.5,606,458 entitled "Head mounted display and viewing system using aremote retro-reflector and method of displaying and viewing an image"hereby are incorporated in their entirety by this reference thereto.

TECHNICAL FIELD

The present invention relates generally, as is indicated, to opticaldisplays and viewing systems, and, more particularly, to such systems inwhich at least part is head mounted or otherwise positioned relative toa viewer and using conjugate optics and accommodation features, and to amethod of displaying and viewing an image.

BACKGROUND

Various types of head mounted displays are known. An exemplary headmounted display (throughout the following specification the initials"HMD" may be used to mean "head mounted display") includes optics oroptical components such as lenses, mirrors or the like, to direct theimage from an image source to an eye or to the respective eyes of aperson viewing the image (viewer). The image source develops and/orprovides an image intended to be viewed and may or may not be part ofthe HMD. Head mounted display systems are used in the field of virtualreality and also in aircraft, for example, as part of a heads-up displaysystem, and in other fields, too. Exemplary use of such display systemsinclude three dimensional (3-D) or stereoscopic display and viewingfeatures. Also, such systems may be used for simulators, such as flightsimulators for pilot training or the like.

Many prior head mounted display systems use light emitting sources tocreate an image, such as a cathode ray tube, light emitting diode, etc.Several disadvantages to such light sources and head mounted displaysusing them are relatively large size, weight, and cumbersome nature. Forexample, in some virtual reality display systems, counterbalancingweights and support systems are needed to hold or to help to hold thehelmet containing the virtual reality image source and optics so that itdoes not severely weigh down the head, neck, shoulders, etc. of theuser.

In some prior display systems a modulator modulates light from a source;the images created are a function of modulation. A liquid crystal cellor liquid crystal display device may be such a modulator. A disadvantageof such modulating systems is the reduction in light output due to lightblocking and/or absorption occurring in the modulator. To overcome suchreduction in brightness, sometimes the intensity of the light source isincreased, which draws additional energy, creates heat, requires alarger light source, etc.

Another disadvantage to prior head mounted display systems is thecomplexity of the components and of the arrangement of the components toprovide the desired display or image output. Complexity, size, and soforth usually increase the cost for such systems and reduce therobustness of the system.

It is desirable that a display, especially an HMD, have adequate eyerelief. and comfort with which images provided by the HMD can be viewed.One aspect of comfort is the distance at which the image is viewed; acomfortable viewing distance is about twenty inches or more, forexample, approximate reading distance. An aspect of eye relief is thedistance between the eye and the last optical element (such as theoutput objective of the display) closest to the eye; often it isdesirable that such distance be relatively large to provide adequate eyerelief. If adequate eye relief is not provided and/or if the viewingdistance at which the image is seen is less than about twenty inches,then the eye may be strained to view the image, which may beuncomfortable and usually is undesirable.

The definition of eye relief also is described in Information Display,Vol. 10, No. 7 & 8, July/August 1994, pages 12-16, the article by RobertE. Fisher entitled "Optics For Head-Mounted Displays."

It would be desirable to reduce the size, weight and complexity of adisplay system, especially a head mounted display system.

It would be desirable to provide a relatively uncomplicated, small androbust display system, especially for an HMD.

It also would be desirable to provide a high quality image, e.g., brightand of good contrast and, if used, color, for viewing using an HMD, andespecially to derive the image using a small display device.

Further, it would be desirable to obtain a relatively wide field of viewin an optical display system, especially a head mounted one, andefficiently to deliver light produced by the light source to the viewer.Efficient delivery of light reduces the brightness requirement of thelight source, energy requirements and output heat, while providing goodbrightness, resolution and contrast of the viewed image.

Additionally, it would be desirable to provide adequate eye relief in ahead mounted optical display system.

Many prior HMDs and other displays have had a limited head box, which isthe size of the area or location at which the viewer's head must beplaced so the eyes see a desired image. For example, if a display weremounted on the head, the eyes are at a location which is relativelyfixed to the place where an image is seen, such as a screen or imagesource, e.g., a projector and/or projection lens, liquid crystaldisplay, CRT display, etc. In some circumstances it would be desirableto enlarge the head box to allow increased movement and positioning ofthe head while still permitting viewing of the displayed image.

Accommodation is the term used to describe the ability of the human eyeto adjust its focus to various distances. This ability is used by thebrain as an added cue in depth perception. If this cue is at variancewith other depth cues such as vergences (stereo parallax) the brain willbe confused, leading to eye strain and an increased susceptibility tomotion sickness. It is therefore, highly desirable in 3-D displaysystems for all of the cues to be in agreement with one another. Theremust also be agreement between the camera system and the display system.The vergence is controlled by the spacing and toe-in of the cameras andprojectors. These can easily be set in agreement. The accommodation iscontrolled by the focus of the camera lens. This can be set to the focaldistance of the objects of interest by one of the standard auto-focusmethods. All that is then required for proper accommodation is that thefocus of the projector be set the same as that of the camera.

SUMMARY

According to one aspect of the invention, a head mounted display systemincludes a retro-reflector, and optical means including an accommodationfeature for directing light from an image source to the retro-reflector,light reflected by the retro-reflector being provided for viewing. Theimage source may be included as part of the HMD.

Briefly, according to the invention, light from an image source isdirected by focusing optics to a conjugate optics path including anaccommodation feature along which the light is directed to the eye oreyes of a viewer for viewing of an image.

In an embodiment of the invention the conjugate optics path is providedby one retro-reflector or more than one retro-reflector which at leastsubstantially maintains the characteristics of the light incidentthereon, including the results of the focusing by the focusing optics,while reflecting the light to the eye(s) of the viewer. In an embodimentof the invention a beamsplitter directs incident light from the focusingoptics into the conjugate optics path, e.g., reflecting light ortransmitting light with respect to the retro-reflector and to the eye(s)for viewing.

Also, light directed to the retro-reflector from the focusing opticsmentioned above is reflected such that the light continues to haveessentially or substantially the same direction it had when it impingedon the retro-reflector so that optically the lens of the eye can appearto be in effect at the focusing optics and the retina of the eye canappear to be in effect at the source of the image.

A further aspect is to use a relatively simple lens system to obtain agood angle of view and comfortable eye relief in an HMD.

Yet an additional aspect is to use a retro-reflector in an HMD, to focuslight from an image source at a location relative to theretro-reflector, and to direct the light at least partly along the pathincident to the retro-reflector for viewing of an image by a viewer,whereby a substantial amount of light from the image source is deliveredto the eye of the viewer.

Even another aspect is to reduce the illumination requirements and/orsize of the image source for an HMD by using an at least partiallyconjugate optical system to direct an image to the eyes of a viewer forviewing.

Even an additional aspect is to reduce the illumination requirementsand/or size of the image source for an HMD by using an at leastpartially conjugate optical system to direct an image to the eyes of aviewer for viewing while providing comfortable eye relief and anaccommodation feature to provide depth characteristics.

According to another aspect, a display system includes means for formingan image for viewing by an eye of an observer, delivery means fordelivering the image from the means for forming to the eye of theobserver, and the means for forming and the delivery means beingcooperative to provide the image to the eye of the observer at a size atthe retina of the eye that is approximately the size of the retina.

According to another aspect, a method of display includes forming animage, and reflecting the image to the eye of an observer such that theimage at the retina of the eye is approximately the size of the retinaof the eye.

One or more of these and other objects, features and advantages of thepresent invention are accomplished using the invention described andclaimed below.

To the accomplishment of the foregoing and related ends, the invention,then, comprises the features hereinafter fully described andparticularly pointed out in the claims. The following description andthe annexed drawings set forth in detail certain illustrativeembodiments of the invention. These embodiments are indicative, however,of but a few of the various ways in which the principles of theinvention may be employed.

Although the invention is shown and described with respect to certainpreferred embodiments, it is obvious that equivalents and modificationswill occur to others skilled in the art upon the reading andunderstanding of the specification. The present invention includes allsuch equivalents and modifications, and is limited only by the scope ofthe claims.

BRIEF DESCRIPTION OF THE DRAWINGS

In the annexed drawings:

FIG. 1 is a schematic illustration of a head mounted display (HMD)optical system in accordance with the present invention shown mounted onthe head of a person relative to one of the eyes;

FIG. 2 is a schematic illustration of an HMD optical system utilizing aretro-reflector;

FIG. 3 is a schematic illustration of the HMD optical system showing therelationship of the size of the image on the retina of the viewing eyerelative to the object to image distance;

FIG. 4 is a simplified schematic illustration of an HMD optical systemsimilar to FIG. 2 showing a comfortable viewing distance for the eye;

FIG. 5 is a schematic illustration of a modified HMD optical systemsimilar to FIG. 2 using an additional lens component to obtaincomfortable viewing distance while reducing the actual size of thesystem;

FIG. 6 is a simplified schematic illustration of an HMD optical systemsimilar to FIG. 2 showing a comfortable viewing distance for the eye;

FIG. 7 is a schematic illustration of a modified HMD optical systemsimilar to FIG. 2 using an additional lens component to obtaincomfortable viewing distance while reducing the actual size of thesystem;

FIG. 8 is a schematic top view of the heads-up/see-through displaysystem of FIGS. 6 and 7 with a portion mounted on the head of a personand a remotely located retro-reflector;

FIGS. 9 and 10 are schematic top views of the heads-up/see-throughdisplay system of FIGS. 6-8 showing limitations on viewing distance forthree dimensional (stereoscopic) viewing;

FIG. 11 is a schematic view of the heads-up/see-through display systemwith automatic focusing adjustment for viewing distance control;

FIG. 12 is schematic illustration of a heads-up/see-through displaysystem utilizing conjugate optics to view an image in a relatively widehead box;

FIG. 13 is a schematic side elevation of the heads-up/see-throughdisplay system of FIG. 12 with a portion mounted on the head of a personand a remotely located retro-reflector;

FIG. 14 is a schematic top view of the heads-up/see-through displaysystem of FIGS. 12 and 13 with a portion mounted on the head of a personand a remotely located retro-reflector;

FIGS. 15 and 16 are schematic top views of the heads-up/see-throughdisplay system of FIGS. 12-14 showing limitations on viewing distancefor three dimensional (stereoscopic) viewing;

FIG. 17 is a schematic view of the heads-up/see-through display systemwith automatic focusing adjustment for viewing distance control;

FIG. 18 is a schematic illustration of an accommodation system useful inthe invention illustrated in the other drawing figures; and

FIG. 19 is a graphical representation of the transfer function for theaccommodation system of FIG. 18.

DESCRIPTION

Referring to the drawings in which like reference numerals designatelike parts in the several figures, and initially to FIG. 1, a headmounted display system (referred to below as HMD) according to oneexample of the invention is shown at 10. The HMD 10 includes a housing11 and a support 12. The HMD is intended to be mounted with respect to aperson who can view images provided by the HMD. An exemplary eye 13 of aperson is shown in FIG. 1 positioned relative to the HMD 10. The HMD mayhave two optical portions (only one being shown in FIG. 1 for simplicityof illustration), each being similar to that shown in the severaldrawings, for respectively providing images to the respective eyes of aperson. The images may be coordinated to provide a three dimensional(3-D) viewing effect (sometimes referred to as stereoscopic viewing), atwo dimensional (planar) viewing effect, or some other effect.Alternatively, one device, such as that shown in FIGS. 1-3, 6 and/or 7,for example, may provide images to both eyes of a person. The HMD 10also may be used to provide images to other devices, such as cameras(still, motion, video, electronic, etc.), measuring equipment, opticalevaluation equipment, copiers, scanners, etc. For illustrative purposesthe HMD will be described in detail with respect to use to provideimages for viewing by a person.

The HMD 10 preferably is relatively small and lightweight to facilitatepositioning relative to a person, and especially to facilitatepositioning relative to the head of a person to provide images to theeyes. In the embodiment illustrated in FIG. 1 the housing 11 is mountedby the support 12 to the head of a person whose eye 13 is depictedschematically. The support 12 may include a strap, cap, temple piece asin eye glasses, or other device to mount the housing 11 on the head orbody of a person or relative thereto to position the housing 11 in frontof the eyes. The support 12 may be part of or may be attached to anothersupport structure mounted on a wall, floor, ceiling, or other structureof a building, vehicle, etc.

The housing 11 contains one or more of the components of the opticalsystem 14 of the HMD 10, as will be described in greater detail below.The optical system 14 may include, as is designated at 15, for example,a light source and/or source of images to be provided by the HMD forviewing, or the housing and optical system may be coupled to anothersource, for example, an external source, such as a liquid crystaldisplay, a cathode ray tube display (CRT), a television, or some othersource, which supplies light and/or images. The housing 11 maycompletely or partially enclose the various optical components of theHMD 10. The housing has an opening or outlet as a viewing port 16through which the eye 13 can view images provided by the HMD.Alternatively, the housing 11 may have one or more additional openingsto receive light and/or a source of images, etc., for deliveryultimately to the viewing port 16. As a further alternative, the housing11 may be one or several structural members which support but do notnecessarily enclose one or more optical components of the HMD so thatthose members are in relative position to provide images for viewing.

In FIG. 2 details of optical components of the optical system 14 of theHMD 10 are shown. The optical components shown in FIG. 2 are similar tothose included in the housing 11 of FIG. 1; however, in FIG. 2 thehousing 11 and support 12 are not shown to facilitate illustrating theinvention and to simplify the drawing.

The optical components 20 of the optical system 14 of the HMD 10 includefocusing optics 21 (sometimes referred to simply as "lens" or asprojection optics or as a projector), a beamsplitter 22 andretro-reflector 23. The HMD also may include an image source 24 (alsodesignated 15 above) which provides images or light havingcharacteristics of an image and, if desired, may be part of thementioned projector. An exemplary image source is a liquid crystaldisplay, such as a small liquid crystal television having across-sectional display area on the order of about one square inch orless. Alternatively, the image source may be separate and simply used toprovide one or more images or light having image characteristics thatcan be provided by the HMD to the eye 13. Additional optical componentsof the optical system 14 may include linear polarizers, circularpolarizers, waveplates, focusing elements, such as lenses or mirrors,prisms, filters, shutters, apertures, diaphragms, and/or othercomponents that may be used to provide a particular type of output imagefor viewing by the eye 13. Examples of several embodiments using suchadditional optical components are described below with respect to otherdrawing figures.

The invention is useful with virtually any type of image source ordisplay source. An example of such a display source is a compact flatpanel display, and especially one utilizing a reflective liquid crystaldisplay made from a single crystal silicon active matrix array. Anexample of such image source is disclosed in copending, commonly ownedU.S. patent application Ser. No. 08/275,907, filed Jul. 5, 1994, theentire disclosure of which hereby is incorporated by reference.

In FIG. 2 the image source 24 displays an image 25, which is shown inthe drawing as an arrow 26. The light 27 leaving the image source 24represents an image or has characteristics of an image, and that lightis collected by the focusing optics 21 of the optical system 14 of theHMD 10 and travels to the beamsplitter 22. In FIG. 2 and in a number ofthe other drawing figures hereof the focusing optics 21 is representedas a single lens. However, it will be appreciated that the focusingoptics 21 may include one or more other components, such as lenses,reflectors, filters, polarizers, waveplates, etc.

Although the image source(s) 24 is shown in the drawings locatedrelatively above the beamsplitter 22, the image source may alternativelybe located below the beamsplitter.

At least some of the light 27a incident on the beamsplitter 22 isreflected by the beamsplitter as light 27b toward the retro-reflector23. The retro-reflector may be, for example, a screen made ofretro-reflecting material. Exemplary retro-reflectors are well known.One example is that known as a corner reflector or a sheet having aplurality of corner reflectors. Another example is a material havingplural glass beads or other refracting and/or reflecting devices on orin a support. An example of a retro-reflector is a film or sheetmaterial having a plurality of comer cubes which material is sold byReflexite Corporation of New Britain, Conn. Such material is availablehaving about forty-seven thousand corner reflectors per square inch.

The light (light rays) 27c, which are shown as broken lines, arereflected by the retro-reflector 23 such that their path is exactly backalong their direction of incidence on the retro-reflector. In this waythe light rays 27c pass through the beamsplitter 22 and are focused,i.e., so as to present a real image in focus as is seen by the light raydiagram of FIG. 2, at a point or location in space, which is generallydesignated 28 in the illustration of FIG. 2. The eye 13 of a viewer(person) can be placed approximately at location 28 to see the image,and the pupil and lens, individually and collectively designated 29, ofthe eye, accordingly, are shown at that point. The lens 29 focuses thelight incident thereon as an image on the retina of the eye 13.

In FIG. 2 the broken lines represent light rays which travel afterreflection by the retro-reflector along the same or substantially thesame path, but in the opposite direction to, respective incident lightrays impinging on the retro-reflector. Thus, the retro-reflector 23 ispart of a conjugate optics path 23a in which light incident thereon isreflected in the same path and opposite direction as reflected light.The beamsplitter 22 directs light from the focusing optics 21 into thatconjugate optics path and toward the retro-reflector; and thebeamsplitter also passes light in the conjugate optics path from theretro-reflector to the output port 16 (FIG. 1) for viewing by the eye13. The beamsplitter 22 and retro-reflector 23 cooperate as a conjugateoptics system to provide that conjugate optics path.

Using the described conjugate optics path and system, relatively minimalamount of the light from the image source 24 and focusing optics 21 islost and, conversely, relatively maximum amount of light is directed tothe eye 13. Also, there is substantial accuracy of image and imageresolution conveyed to the eye. Furthermore, especially if a relativelygood quality retro-reflector is used so that the precise location atwhich the image 30 is in focus will not be critical, e.g., it can bebehind or in front of the retro-reflector, the tolerance required forthe relative positioning of the components of the optical system 14 isless severe. This makes the HMD 10 relatively robust and reliable.

In FIG. 2 the viewed image 30 is represented by an enlarged arrow 31.Such arrow 31 is shown in FIG. 2 as a magnified focused image of theimage 25 from the image source 24. The image 30 may be in focus at orapproximately at the retro-reflector 23, and this is especiallydesirable for good quality images to be provided the eye 13 when arelatively low quality retro-reflector is used. A low qualityretro-reflector is one which has relatively low resolution or accuracyof reflecting light in a conjugate manner in the same path but oppositedirection relative to the incident light. With a low or poor qualityretro-reflector and the image not being focused at the retro-reflector,it is possible that too much light may be lost from the desiredconjugate optics path back to the eye 13, and this can reduce thequality of the image seen. However, the image 30 may be in focus atanother location or plane either behind the retro-reflector (relative tothe eye) or in front of the retro-reflector, and this is easier to dowhile maintaining a good quality image for viewing when theretro-reflector is a good quality one. The better the retro-reflector,the more self-conjugating is the optical system 14 and the less the needto focus with precision at the retro-reflector.

Retro-reflector quality may be indicated by the radians of beam spreadof light reflected. For example, a relatively good qualityretro-reflector may have from zero or about zero radians of beam spreadto a few milliradians of beam spread. The quality usually is consideredas decreasing in proportion to increasing beam spread of reflectedlight.

In considering the brightness of the image seen by the viewer, thenature of the beamsplitter 22 plays a role. The light produced by theimage source 24 may be polarized or unpolarized. If the beamsplitter 22is of a non-polarizing type, then a balanced situation is to have 50% ofthe light incident on the beamsplitter 22 be reflected and 50%transmitted. Thus, of the light 27a incident on the beamsplitter 22, 50%is reflected and sent toward the retro-reflector screen 23 as light 27b.Of the reflected light 27c from the retro-reflector 23, 50% of the lightwill be transmitted through the beamsplitter 22 and will travel to theviewer's eye 13. This configuration of the optical components 20 of theHMD 10 can transfer to the viewer's eye a maximum of 25% of the lightproduced by the image source 24. If desired, the beamsplitter 22 can bemodified in ways that are well known to change the ratio of thereflected light to transmitted light thereby. Also, the beamsplitter 22may include an anti-reflection coating so that all or an increasedamount of the image comes from one side of the beamsplitter and thus toreduce the likelihood of a double image.

Since the optical system 14 of the HMD 10 provides good resolution ofthe image and maintains the characteristics thereof, the image sourcecan be a relatively inexpensive one that does not have to compensate forsubstantial loss of image quality that may occur in prior HMD systems.Furthermore, since a relatively large amount of the light provided bythe image source 24 is provided to the eye 13 for viewing, e.g., sincethe retro-reflector can virtually focus the light for viewing at theeye, additional brightness compensation for loss of light, as may beneeded in prior HMD systems, ordinarily would not be required.

For exemplary purposes, in FIG. 2 three light rays 40a, 40b, 40c(collectively 40) originating at the tip of the arrow 26 constitute aportion of the light 27. Three light rays schematically shown at 41a,41b, 41c (collectively 41) also are examples of light emanating at thetail of the arrow 26. The light 27 has characteristics of the image 25from or provided by or at the image source 24, and represented by theexemplary light rays 40 and 41, is focused by the focusing optics 21onto the retro-reflector 23. The size of the image 30 seen as the arrow31 on the retro-reflector 23 depends on the focal length of the focusingoptics 21 and the distances between the image source 24 and theretro-reflector 23 from the focal points 43, 44 of the focusing optics21. The image source 24 should be located relative to the focusingoptics 21 such that an image can be focused, e.g., in focus as is shownin FIG. 2, at or approximately at the retro-reflector. For example, theimage source 24 may be beyond the focal point 43 of the focusing optics21, and the retro-reflector likewise preferably is beyond the focalpoint 44 of the focusing optics so that the image can be focused at theretro-reflector.

In the illustration of FIG. 2 the image 30 on the retro-reflector 23 ismagnified relative to the size of the image at the image source 24; itdoes not have to be magnified. The image 30 may be the same size as theimage 25 or it may be smaller. Thus, although the image source 24 may berelatively small and/or may provide a relatively small size image 25 atits output, the size of the image 30 viewed by the eye 13 may bedifferent.

The optical system 14 is operable to place the image plane effectivelyat the retina of the viewer's eye 13. This is accomplished byeffectively putting the plane of the eye lens (or pupil) 29 effectivelyat the position occupied by the focusing optics 21 relative to thesource of the image provided to the focusing optics. In a sense the lens21 is optically superimposed on the lens 29 of the eye 13.

The invention provides an optical system in which there are conjugatepaths from a lens, such as focusing optics 14, which corresponds to the"lens means" of an optical sensor, e.g., the eye 13. Stated in anotherway, the invention presents visual information with a wide field of viewby taking the output from a lens (focusing optics 21) and reflecting thelight back onto the same lens, but actually direct that reflected lightonto the eye. This is obtained by using the conjugate optics arrangementdisclosed herein.

Turning to FIG. 3, the arrangement of optical components in the opticalsystem 14 of the HMD 10 is presented to demonstrate the obtaining of awide field of view in accordance with the invention. As is seen in FIG.3, exemplary light rays 40b, 41b originate from the respective ends ofthe image 25 arrow 26 at the image source 24, and light ray 50originates from the center of the arrow 26. These light rays 27a passthrough the focusing optics 21, here shown as a lens, are reflected bythe beamsplitter 22, and are incident on the retro-reflector screen 23as light 27b. The light rays are retro-reflected by the retro-reflectorscreen 23 as light 27c so that they converge, i.e., are focused, at apoint 28 in space. As is seen in FIG. 3, when the pupil of a viewer'seye 13 is placed at this point 28 in space, lens 29 focuses the imageonto the retina and it is possible to see the image 30.

To obtain a relatively wide field of view the entire retina 51 of theeye 13 may be filled with the image, shown at 52 in FIG. 3. The more theretina is filled, the wider will be the field of view. In theillustrated HMD 10 the magnification of the image 52 on the retina 51equals the object distance between the image source 24 and the focusingoptics 21, for example, which is shown at 52 in FIG. 3, divided by theimage distance shown at 54. The image distance, of course, is thedistance between the lens 55 of the eye 13 and the retina 51.

Also, in the arrangement of optical components 20 of the HMD asillustrated in FIG. 3, the pupil 30 of the eye 13 is located at adistance from the beamsplitter 22. This distance, which is usuallyconsidered in relation to "eye relief", also is determined by the f# ofthe focusing optics 21. By adjusting the parameters, such as f# andobject distance, the eye relief can be made at any distance that iscomfortable and convenient.

In operation of the HMD 10, light having characteristics of an image isprovided by the image source 24. The focusing optics 21 directs thelight to the beamsplitter 22, which reflects the light to theretro-reflector 23 for focusing approximately at or near theretro-reflector. Light is reflected by the retro-reflector to transmitthrough the beamsplitter and to reach the eye 13. The eye lens 29focuses the light onto the retina 51 to see the image.

The back of the eye 13, e.g., the retina, in effect is at the imagesource 24 relative to the focusing optics 21, and the lens 29 of the eyein effect is at the focusing optics relative to the image source. Thus,the lens 29 of the eye stands relative to the image source 24 in thesame relation as the lens of the focusing optics 21 stands relative tothe image source. One has a full field of view as provided by thefocusing optics. The location of the image seen by the eye 13 is afunction of the plane at which the focusing optics focuses its projectedimage.

According to the invention, the HMD uses a retro-reflector, and thefocusing optics 21 focuses the image from the image source at orapproximately at the retro-reflector. Thus, a large amount, for example,substantially all the light from the focusing optics, can be directed bythe retro-reflector and put at the eye 13. Of course, there may be otherlosses in the various components of the optical system, and there may beloss of light due to transmission of the incident light from thefocusing optics onto the beamsplitter; and there may be reflection oflight from the retro-reflector by the beamsplitter back toward thefocusing optics and image source. Some of these losses are addressed inthe several embodiments described further below. However, in view of theefficiency of the optical system 14 of the invention, only relativelylow illumination level may be needed from the image source.

Using the invention it is possible to magnify the image from arelatively small display to create a suitable image for viewing withouthaving to place the optics and/or image source too close to the eyes,and, therefore, comfortable eye relief can be obtained. Furthermore,using a relatively simple and uncomplicated focusing optics 21, such asa single lens or a camera lens system, such as that used in aconventional SLR camera, it is possible to obtain a relatively goodangle of view and good, comfortable eye relief. Using the invention itmay be unnecessary to use an output objective or other objective that isplaced rather close to the eye of the viewer while still providing anacceptable image for viewing.

For some magnification situations, as is described further in detailbelow, it is desirable to use a relatively short focal length focusingoptics 21 such that the focal length thereof is less than that of thelens 29 of the eye 13 to the back of the eye. Also, to provide arelatively large or wide "sweet spot" or place where the eye 13 can bepositioned relative to the optical system 14 and/or the output port 16,while still being able to see a good quality (bright, good resolution,and/or good contrast, etc.) image preferably also with a relatively widefield of view, it is desirable to use a relatively short focal lengthlens or focusing optics 21, and even more preferably to use such afocusing optics 21, indeed, optical system 14 overall, which has arelatively low f#.

It may be desirable to use lens (focusing optics) 21 having a relativelylow f# and short focal length. The exit aperture size of the image lens21 determines the size of the sweet spot. The optical system 14 canchange the field of view between relatively wide and relatively narrowby changing the focal length of the focusing (projection) optics 21.

Summarizing the operation of the invention depicted in FIGS. 2 and 3,for example, light representing the image 25 provided by an imagesource, such as that shown at 24, is directed via the focusing optics 21and beamsplitter 22 to the retro-reflector 23 to form a real image at ornear the retro-reflector. Preferably a broad image field is focused atthe retro-reflector. Light reflected by the retro-reflector 23 isdirected toward the location 28 via the beamsplitter 22, and the lens 29of an eye 13 may be placed at the location 28 to receive light reflectedby the retro-reflector. The retina 51 (FIG. 3) is located at the imageplane of the lens 29 which focuses the light there to form the viewed orseen image.

The optical components 20 used in the optical system 14 of the inventionare operative in a sense as a viewer or in other words like an eyepieceor objective for use to view an image. The optical components 20 areable to provide the viewing or eyepiece function usually independentlyof the light source or source of images intended for viewing. Therefore,although the focusing optics 21 may provide the function of projectingan image or directing an image toward the retro-reflector 23, it will beappreciated that the invention may be considered, at least in part, as aviewer or a technique for viewing images rather than as a conventionalprojector which projects images onto a conventional movie screen. Theinvention may effectively make the image plane reflective to the eye(s)of a viewer; for example, the retro-reflector 23 reflects the lightforming the real image toward the eye 13 which is able to view theimage.

The size of the exit pupil of the optical system 14 as provided to theeye 13 is proportional to the diameter of the lens or focusing optics21. If there are no other focusing type (lenses or mirrors, for example)optical components in the optical system 14 other than the lens 21, thenthe exit pupil of the optical system 14 will be the same as the diameterof the lens or focusing optics 21. Magnification is proportional to thefocal length of the lens or focusing optics 21. However, if the focallength of the lens 21 is too short, the lens 21 and the exit pupil ofthe optical system 14 will not cover the entire field of the imagesource 24.

The location at which the image, which is provided by or viewed via theoptical system 14, by the eye is a function of the plane at which thelens 21 focuses its projected image. That location can be at theretro-reflector 23, which is preferred if a relatively low qualityretro-reflector is used; or that location can be behind or in front ofthe retro-reflector, which is especially possible when a relatively goodquality retro-reflector is used.

The invention may be used to present a real image over a relatively widefield. For example, the angle of the field of view may be on the orderof about 100 degrees using only a single retro-reflector. This is incontrast to many conventional heads up displays in which there is arelatively narrow field of view of an image, for example, on the orderof 10-30 degrees, which is superimposed or adjacent part of a broaderreal view and which is image essentially focused at infinity.

Furthermore, it will be appreciated that although the invention isdescribed and illustrated herein as having the beamsplitter reflectlight from the image source to the retro-reflector; and then thebeamsplitter transmits light from the retro-reflector to the eye, otherarrangements of the optical components also are possible. An example ofanother such arrangement would have the image source, beamsplitter andretro-reflector in a straight line so the beamsplitter transmits lightfrom the image source and focusing optics to the retro-reflector; andlight that is reflected by the retro-reflector is reflected by thebeamsplitter to the eye for viewing.

The human eye is most comfortable when viewing an image at a distance ofabout twenty inches, approximately at the distance at which one wouldplace a book, document, etc. to be read. It is desirable that the finalimage as seen by the viewer be located at such distance, e.g.,approximately twenty inches from the pupil 29 of the eye. This can beaccomplished in the manner illustrated in FIG. 6. The retro-reflector 23on which the focused image from the image source 24 is moved to twentyinches from the pupil 29 of the eye 13. A disadvantage with this system,though, is the large size of the HMD 10 required to provide suchdistance between the eye and the retro-reflector.

The HMD can be made more compact compared to the HMD illustrated in FIG.6 by adding an additional optical system 70 between the beamsplitter 22and the eye 13. In FIG. 7 such optical system 70 is depicted as a singlelens; however, it will be appreciated that it may include other opticalcomponents as was mentioned above, for example, with respect to thefocusing optics 21. In the HMD 10 of FIG. 7 the retro-reflector 23 iscloser to the eye 13 than it is in FIG. 6 making a more compact system.The viewer is provided with a virtual image 30' of the image source 24at the desired viewing distance (twenty inches, for example) by thecooperation between the focusing optics, retro-reflector, and additionaloptical system 70.

In the example of FIG. 7, the additional viewing lens 70 may be, forexample, about 10 diopters. To present to the eye 13 an image whichappears at a comfortable viewing distance, such as about 20 inches ormore away, the lens 70 may be located from about 1/2 to 1 inch in frontof the eye. Although in many viewing devices further spacing between theeye and the optical component of the optical system nearest the eye maybe desired to obtain desired eye relief, the use of lens 70 at theindicated distance of about 1/2 to 1 inch from the eye usually isacceptable and reasonably comfortable because that is the approximatespacing of ordinary eye glasses to which people ordinarily relativelyeasily become accustomed.

The function of the lens 70 may be obtained by using a negative lens atthe focusing optics 21.

Referring, now, to FIGS. 12-17, another embodiment of display andviewing system 210 in accordance with the invention is illustrated. Thevarious components and features of the invention described in theseveral embodiments presented above may be used in the embodiment ofFIGS. 12-17, as will become further evident from the description below.In FIGS. 12-17 the same reference numerals as were used above todesignate similar parts in the embodiments of FIGS. 1-11 are used withthe addition of the value 200. Therefore, reference numerals 210, 213,etc., generally corresponds to reference numerals 10, 13, etc.,described above.

The display and viewing system 210 in a sense is a hybrid head mounteddisplay in that the housing 211 contains or supports the focusing optics221, beamsplitter 222 and image source 224 and may be mounted on thehead or in a position where the head can be placed relatively nearthereto to view an image, and the retro-reflector 223 is relativelyremotely located and does not necessarily have to be located in, on orattached with respect to the housing 211. The optical system 214includes the focusing optics 221, beamsplitter 222 and retro-reflector223, but the retro-reflector may be located relatively remotely withrespect to those other components of the optical system.

In using the display and viewing system 210, light 227a shown in solidlines from the image source 224 is directed from the focusing optics 221via the beamsplitter 222 and then as light 227b toward theretro-reflector 223; and that light is so focused as to form a realimage at the retro-reflector or in front or behind the retro-reflector.Since the retro-reflector 223 may have some dispersion or beam spreadingcharacteristics, e.g., it is not perfect, and since the retro-reflectormay be relatively far from the beamsplitter and focusing optics, it maybe desirable to focus the real image at the retro-reflector, e.g., inthe plane thereof if the retro-reflector is flat. The retro-reflector223 reflects the incident light in a conjugate path 223a as light 227ctoward the beamsplitter 222 via which the light reaches the eye 213, asis shown by the dotted lines in FIG. 12, for example.

The housing 211 may include a support 300 for supporting a plurality ofimage sources 224L (left) and 224R (right), and associated focusingoptics 221L and 221R for directing light via a shared beamsplitter 222or separate respective beamsplitters along the path represented by light227b. It will be appreciated that the beamsplitter 222 may be a singleone having shared by both image sources and focusing optics withrespective left and right eye light paths reflected by or transmittedthrough respective left and right portions of the beamsplitter; or thebeamsplitter may be divided as two separate ones, each positioned in arespective one of the light paths to operate in the manner describedherein. In FIG. 14 the left eye 213L image is provided by the imagesource 224L and the right eye image is provided by the image source224R; light from those image sources is shown directed along respectivelight paths 227bL and 227bR.

The housing 211 and support 300 may be mounted on the head 301 of aperson by appropriate headgear 302 shown in FIGS. 13-16, for example.Various types of headgear are known in the art and may be used for thispurpose. It is desirable to minimize the mass and size of the headgear,and, therefore, a rather simple form such as that shown usingcircumferential and top straps may be used. Size adjustments 303 may beused, as is shown schematically in FIG. 13. This is consistent with theuse of a relatively remotely located retro-reflector which does not haveto be supported on the headgear 302 or housing 211 in this embodiment.Since the retro-reflector is relatively remotely located the size andweight of the housing 211 and of those portions of the optical system214 which would have to be otherwise supported on or with respect to thehead of a person can are reduced relative to the embodiments describedabove with respect to FIGS. 1-11, for example.

An electrical cable 304 may provide electrical power and image signalsto the respective image sources 224L, 224R. Other means may be used forthese purposes. For example, a battery power supply may be mounted onthe headgear 302 and/or housing 211; and other means may be used tocouple image or information signals to the image sources 224, e.g.,radio or optical transmission, etc. The image signals may be provided byoptical cable or some other means from a remote source and those opticalimage signals may be directed to the focusing optics 221 for forming areal image at the retro-reflector 223. Various other equivalent meansmay be used to provide power and a source of image signals.

The embodiment of display and viewing system 210 as a number ofadvantages and distinctions from the system 10. The head box becomesinherently large. It is possible to look directly through thebeamsplitter 222 to see images and objects essentially the same as onewould see them when looking through a clear medium, such as glass orplastic. As will be evident from the description hereof, images from theimage sources 224 will be directed to the eyes of the person orapparatus wearing or using the system 210 only when light 224b isincident on a retro-reflector. That portion of the system 210 whichwould be mounted on the head is relatively lighter weight because thereis no need to support the retro-reflector thereon.

When a user looks in a direction other than at a retro-reflector, theimage(s) from the image source(s) 224 will not be returned and all thatis seen is a view of the "outside world". Additionally, the headgear 302and system 210 not only permit an unobstructed view of the outsideworld, but also accomplishes this with enough eye relief to allow theuser easily to wear eye glasses. This is particularly beneficial if theuser requires bifocals, in which case if the user looks down, thebeamsplitter 222 is not encountered at all. Therefore, the bifocal eyeglasses permit the user to perform close-up tasks as normal and, ifdesired, to look up through the beamsplitter toward a retro-reflectormaterial to see the image(s) originating from the images source(s) 224.

The image reflected by the retro-reflector 223 for viewing by a user ofthe system 210 is relatively private due to the conjugate optical pathsalong which light travels from the image source(s) via thebeamsplitter(s) to the retro-reflector and then back via theretro-reflector to the eye(s) of the user. The retro-reflectorpreferably has relatively little beam spread so other people in a room,for example, may not even know that a user is seeing an image reflectedby a retro-reflector in the room.

In using the system 210, a user wears the housing 211 on which the twoimage sources 224L, 224R, which may be considered, in combination withthe respective focusing optics 221, projectors 305 (e.g., 305L, 305R forleft and right eye images, respectively) in view of the fact that theyproject light. The headgear 302 may be adjusted for comfortable fit. Aretro-reflector is placed anywhere the user wishes to see an image. Forexample, if the user is sitting at a desk, the retro-reflector may beplaced flat on the desk similar to a piece of paper. Alternatively, theretro-reflector could be oriented in a position similar to the front ofa computer monitor or other type of display device. The user would seethe image only when looking at the retro-reflector. It is only in thiscircumstance that the light emitted by a projector(s) (image source 224and focusing optics 221) would be returned to the user's eye(s). Whenthe user looks in another direction, the projector image is notreturned, and all that is seen is a view of the outside world.

Since there are two projectors 305L, 305R, the image can be stereoscopicor three dimensional. To get a good quality stereoscopic imagery, it isnecessary that the right eye image not be seen by the left eye and viceversa. In order to assure that this is the case consideration may begiven to the design of the system 211, as follows. The light returnedfrom the retro-reflector 223 actually has some small divergence, e.g.,on the order of a few milliradians, as the retro-reflector likely wouldnot be a perfect retro-reflector that reflects all incident lightprecisely along respective perfect conjugate optical paths. This spreadin the reflected or returned light beam means that there is a maximumdistance between the user and the retro-reflector for which the imagesto the eyes do not overlap, as is illustrated in FIG. 15. In theillustration of FIG. 15, it will be appreciated that all light emanatingfrom the left projector 305L and directed as light 227bL toward theretro-reflector is reflected by the retro-reflector 223 as light 227cLto the left eye 13L; and even though there is some beam spread of lightreflected by the retro-reflector, none of the spread light reaches theright eye 13R. Similarly the light from the right projector 305R isdirected to the right eye 13R and does not reach the left eye.

However, as is illustrated in FIG. 16, there is relatively more beamspread that occurs in the light reflected by the retro-reflector 223because the retro-reflector is further from the housing 211 than is thecase in the illustration of FIG. 15. The increased beam spread also oradditionally may be due to a lower quality retro-reflector in theillustration of FIG. 16 than that used in the illustration of FIG. 15.In FIG. 16 it is seen that light 227bL (the light envelope of which isrepresented solid lines) is directed from the left projector 305L to theretro-reflector 223. However, the light reflected by the retro-reflector223 as light 227cL spreads in a sufficiently wide pattern that some isreceived by the left eye 13L but some also is received by the right eye13R. Similarly, in the illustration of FIG. 16 light from the rightprojector 305R may be spread sufficiently wide as to be received uponreflection by the retro-reflector 223 also by the left eye 13L.

The overlapping images degrades the operation of the system 210 forstereoscopic image viewing. Therefore, when the viewed images are toprovide stereoscopic views, it is desired that the left and right eyeimages would not overlap. This can be accomplished, for example, byusing a relatively good quality retro-reflector with small beam spreador by locating the retro-reflector remotely of the housing 211 butsufficiently close so that the beam spread does not result in imageoverlap.

For the user to see a relatively sharp image it usually is necessary forthe projector(s) 305 to be focused on the plane of the retro-reflector223, e.g., to focus a real image there. If the retro-reflector to userdistance is relatively fixed, then the focus of the system 210 can beset up at the time of initial use. However, if the situation is suchthat the system 210 is to be used in a variety of retro-reflector touser distance conditions, then a manual adjustment can be provided toadjust the focus of both focusing optics systems 221L and 221Rseparately or simultaneously in order to obtain the desired focusingmentioned. Such manual focus control adjustment 309 is shownschematically in FIG. 17.

As also is shown in FIG. 17, an automatic focus control can be provided.An automatic focus control device, such as that used in a conventionalautomatic focusing camera, may be used for this purpose as is indicatedat 310. The automatic control 310 may include an ultrasonic emitter anddetector 311 which may be mounted on the headgear 302, for example, orelsewhere to provide a measurement or detection of the distance of theretro-reflector from the focusing optics 221L, 221R. The time requiredfor a round trip pulse allows the calculation of the distance. Thefocusing optics or lenses 221L, 221R can be adjusted to proper focus bythe focus control 312 according to such distance information. Othertypes of distance measuring and/or control devices also may be used inaccordance with this focusing feature of the invention.

Although the system 210 shown in FIGS. 12-17 show use of two projectors305L, 305R, it will be appreciated that a single projector may be usedto develop the images. The images may be provided in field sequentialfashion to alternate eyes using optical dividing devices, such asreflectors and beamsplitters, not shown, in order to obtain stereoscopicimages. Alternatively, the images to the left and right eyes may beessentially the same and produced either in field sequential manner orsimultaneously to obtain a planar (non-stereoscopic) image for viewingin the manner described herein.

Accommodation may be used, for example, in the system 210 of FIGS.12-17. Accommodation describes the ability of the human eye to adjustits focus to various distances. This ability is used by the brain as anadded cue in depth perception. If this cue is a variance with otherdepth cues such as vergences (stereo parallax) the brain will beconfused, leading to eye strain and an increased susceptibility tomotion sickness. It is, therefore, desirable in 3-D display systems forall of the cues to be in agreement with one another. There also shouldbe agreement between the camera system and the display system. Thevergence is controlled by the spacing and toe-in of the cameras whichpick up the images and projectors, such as those at 305L and 305R, whichdisplay the images from the cameras for viewing. These can easily be setin agreement. The accommodation is controlled by the focus of the cameralens. This can be set to the focal distance of the objects of interestby one of the standard auto-focus methods. All that is then required forproper accommodation is that the focus of the respective projector(305L, 305R, for example) be set the same as that of the camera.

An auto-focusing lens, such as that conventionally used with CCDcameras, takes an electronic signal generated by the camera and uses itto control a servo motor inside of the lens. This motor controls theposition of the optical elements of the lens, which in turn determinesthe distance to the object plane. The images of objects which arelocated at this distance, will be in focus at the CCD element. Thealgorithm for generating this signal is built into the hardware of thecamera and is commercially available.

The signal which controls the lens on the camera can also be used tocontrol the projection lens of a conjugate optics projector, such aslenses 221L, 221R, for example. If the same focal length lens is usedfor both the camera and the projector, then signal can be fed directlyto both lenses. The lens to LCD (224L, 224R, for example) distance ofthe projector will be servo locked, equal to the lens to CCD distance ofthe camera. As a result, the image distance for the projector willalways be the same as the object distance for the camera. In general,because the CCD and LCD elements are not the same size, different focallength lenses will be used on the camera and projector. This willrequire some electronic processing of the signal going to the projectorlenses 221L, 221R, for example, in order to correctly scale theirpositions, and such processing may be easily carried out.

In order for the user of the conjugate optics system 210, for example,including an accommodation feature, to see the projected image in focus,the retro-reflector 223 should be placed reasonably close to the imageplane. If the retro-reflector is located at fixed distance from theprojection lens, the useful range will be restricted. Alternatively, theposition of the retro-reflector could be changed to follow the focalplane of the projector. This would be difficult to do with a remoteretro-reflector. But, if an extra lens 400, called a conjugation lens,is added between the beamsplitter 222 and the retro-reflector 223, thenthe retro-reflector can be moved quite close to the projector. See FIG.18 for an illustration of the geometry. An image of the LCD 224 isfocused at a point 401 in the far field by the projection lens 221.Normally the retro-reflector would be located at this point. When theconjugation lens 400 is introduced, it refocuses the image to a point402 much closer. The retro-reflector is now placed at this new imageplane. The nature of the retro-reflector 223 is to return any ray oflight impinging on it, back along the same path to its source. As itpasses back through the conjugation lens 400, the ray is returned to thesame path it would have followed had the retro-reflector been placed atthe original image plane and the conjugation lens removed. The viewersees the same image with either system. The advantage of the closeretro-reflector of FIG. 18 is that the refocused image plane moves onlya short distance as the original image plane moves from infinity to thenear field. This short travel means that the position of theretro-reflector 223 can be easily controlled by a servo which is linkedback to the servo that controls the projection lens 221. Therelationship (sometimes referred to as a transfer function) between theprojector lens image distance and the retro-reflector position is givenby the formula: ##EQU1## Where: D=The distance between the conjugationlens and the retro-reflector (see FIG. 18)

C=The distance between the conjugation lens and the LCD (see FIG. 18)

F₁ =The focal length of the projection lens

F₂ =The focal length of the conjugation lens

R=The distance projection lens and its image focal plane (see FIG. 18)

FIG. 19 provides a graphical example of this transfer function.

In view of the foregoing, it will be appreciated that the presentinvention may be used to display images for viewing by using conjugateoptics and accommodation techniques.

I claim:
 1. A display system including a retro-reflector, and opticalmeans including an accommodation feature for directing light from animage source to the retro-reflector, said optical means includingfocusing optics for focusing an image from the image source relative tothe retro-reflector, light reflected by the retro-reflector beingprovided for viewing, said optical means and retro-reflector beingcooperative to establish a conjugate optics path including theaccommodation feature along which the light is directed to the eye oreyes of a viewer for viewing of an image, and a beamsplitter in theconjugate optics path.
 2. The system of claim 1, said retro-reflectorcomprising plural retro-reflectors.
 3. The system of claim 1, whereinsaid retro-reflector at least substantially maintains the characteristicof the light incident thereon, including the results of focusing by thefocusing optics, while reflecting the light to the eye(s) of the viewer,and wherein said optical means includes means for providing pluraloptical paths to provide a stereoscopic or three dimensional image. 4.The system of claim 3, wherein said beamsplitter is positioned to directlight from the focusing optics into the conjugate optics path byreflection or transmission of light toward the retro-reflector and fromthe retro-reflector to the eye(s) for viewing.
 5. The system of claim 1,wherein light directed to the retro-reflector from the focusing opticsis reflected such that the light continues to have substantially thesame direction it had when it impinged on the retro-reflector so thatoptically the lens of the eye can appear to be in effect at the focusingoptics and the retina of the eye can appear to be in effect at thesource of the image.
 6. The system of claim 1, wherein plural imagesources respectively provide images from respective image acquiringdevices positioned and focused to represent depth information, saidoptical means includes means for providing plural optic al paths toprovide a stereoscopic or three dimensional image, and saidaccommodation feature being operative to provide depth characteristicfor the viewed image.
 7. The system of claim 1, said accommodationfeature comprising at least one focusing device.
 8. The system of claim7, said at least one focusing device comprising a lens.
 9. The system ofclaim 8, said focusing device comprising an auto-focusing device. 10.The system of claim 7, said optical means comprising a projection lensmeans for projecting an image from a source toward the retro-reflector,further comprising a camera lens for obtaining an image for projection,and said at least one focusing device of said accommodation featurecomprising means for at least substantially maintaining equal theeffective focus of the projection lens means and the camera lens. 11.The system of claim 10, said retro-reflector being positioned inproximity to the image plane of the projection lens means including theaccommodation feature.
 12. The system of claim 11, further comprisingmeans for changing the position of the retro-reflector to follow thefocal plane of the projection optics.
 13. A display system including aretro-reflector, and optical means including an accommodation featurefor directing light from an image source to the retro-reflector, lightreflected by the retro-reflector being provided for viewing, saidoptical means and retro-reflector being cooperative to establish aconjugate optics path including the accommodation feature along whichthe light is directed to the eye or eyes of a viewer for viewing of animage,said accommodation feature comprising at least one focusingdevice, said optical means comprising a projection lens means forprojecting an image from a source toward the retro-reflector, furthercomprising a camera lens for obtaining an image for projection, and saidat least one focusing device of said accommodation feature comprisingmeans for at least substantially maintaining equal the effective focusof the projection lens means and the camera lens, said retro-reflectorbeing positioned relatively close to the image plane of the projectionlens means including the accommodation feature, and further comprising abeam splitter to direct light from the focusing optics into theconjugate optics path by reflection or transmission of light toward theretro-reflector and from the retro-reflector to the eye(s) for viewing.14. The system of claim 13, said accommodation feature comprising aconjugation lens between the beamsplitter and the retro-reflector.
 15. Adisplay system comprising a display source, optics for focusing an imagefrom the display source in the far field using a projection lens, aretro-reflector in proximity to which such image is focused, andconjugation lens means to refocus the image to a point much closer tothe optics, whereby to permit the retro-reflector to be positioned atthe closer image point.
 16. The system of claim 15, wherein theconjugation lens means and retro-reflector are cooperatively positionedsuch that as light passes back through the conjugation lens means alongthe same path it would have followed had the retro-reflector been placedat the image plane of the projection lens had the conjugation lens meansnot been in position.
 17. The system of claim 16, further comprising aservo system for moving the projection lens and the retro-reflectoraccording to a transfer function.
 18. The system of claim 17, whereinthe transfer function is given by the formula: ##EQU2## where: D=Thedistance between the conjugation lens and the retro-reflector (see FIG.18)C=The distance between the conjugation lens and the LCD (see FIG. 18)F₁ =The focal length of the projection lens F₂ =The focal length of theconjugation lens R=The distance projection lens and its image focalplane (see FIG. 18).
 19. A method for displaying an image for viewingcomprising focusing an image from a display source in the far fieldusing a projection lens, a beamsplitter, a retro-reflector, andconjugation lens means to refocus the image to a point much closer tothe optics than the far field image point of the projection lens,whereby the retro-reflector may be positioned at the closer image point,wherein the conjugation lens means and retro-reflector are cooperativelypositioned such that as light passes back through the conjugation lensmeans along the same path it would have followed had the retro-reflectorbeen placed at the image plane of the projection lens had theconjugation lens means not been in position, and using a servo systemfor moving the projection lens and the retro-reflector according to atransfer function given by the formula: ##EQU3## where: D=The distancebetween the conjugation lens and the retro-reflector (see FIG. 18)C=Thedistance between the conjugation lens and the LCD (see FIG. 18) F₁ =Thefocal length of the projection lens F₂ =The focal length of theconjugation lens R=The distance projection lens and its image focalplane (see FIG. 18).
 20. A display system including a retro-reflector,and optical means including an accommodation feature for directing lightfrom an image source to the retro-reflector, light reflected by theretro-reflector being provided for viewing, said optical means andretro-reflector being cooperative to establish a conjugate optics pathincluding the accommodation feature along which the light is directed tothe eye or eyes of a viewer for viewing of an image,said accommodationfeature comprising at least one focusing device, said optical meanscomprising a projection lens means for projecting an image from a sourcetoward the retro-reflector, said retro-reflector being positionedrelatively close to the image plane of the projection lens meansincluding the accommodation feature, and further comprising a beamsplitter to direct light from the focusing optics into the conjugateoptics path by reflection or transmission of light toward theretro-reflector and from the retro-reflector to the eye(s) for viewing.